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Phases and excitations of active rod–bead mixtures: simulations and experiments

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Abstract

We present a large-scale numerical study, supplemented by experimental observations, on a quasi-two-dimensional active system of polar rods and spherical beads confined between two horizontal plates and energised by vertical vibration. For a low rod concentration Φr, our observations are consistent with a direct phase transition, as bead concentration Φb is increased, from the isotropic phase to a homogeneous flock. For Φr above a threshold value, an ordered band dense in both rods and beads occurs between the disordered phase and the homogeneous flock, in both experiments and simulations. Within the size ranges accessible, we observe only a single band, whose width increases with Φr. Deep in the ordered state, we observe broken-symmetry “sound” modes and giant number fluctuations. The direction-dependent sound speeds and the scaling of fluctuations are consistent with the predictions of field theories of flocking; sound damping rates show departures from such theories, but the range of wavenumbers explored is modest. At very high densities, we see phase separation into rod-rich and bead-rich regions, both of which move coherently.

Graphical abstract: Phases and excitations of active rod–bead mixtures: simulations and experiments

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Article information


Submitted
31 Dec 2019
Accepted
18 Apr 2020
First published
20 Apr 2020

Soft Matter, 2020, Advance Article
Article type
Paper

Phases and excitations of active rod–bead mixtures: simulations and experiments

H. Soni, N. Kumar, J. Nambisan, R. K. Gupta, A. K. Sood and S. Ramaswamy, Soft Matter, 2020, Advance Article , DOI: 10.1039/C9SM02552A

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